1. Field of the Invention
The present invention relates to a film formation method for casting a ribbon including polymer solution on a support from an extruder die. More specifically, the present invention relates to a film forming method in which the ribbon is cast on the support without fluctuation.
2. Background Arts
A cellulose triacetate film can be used as a support for a photosensitive material. The cellulose triacetate film is formed by a liquid film formation method, in which a filmy polymer solution in an organic solvent is cast onto the moving support from an extruder die. The filmy polymer solution ranged from a nozzle of the extruder die to the support is called as ribbon. In order to fix the ribbon on the support and to prevent introduction of the air into the polymer solution, JP-A 2-52721, JP-A 6-155494 and JP-A 10-264185 disclose a film forming method in which a decompression area is formed behind the extruder die by a decompression device. For the purpose of preventing introduction of the air, which is thought to vibrate the ribbon, it is also possible to provide side decompression area to reduce pressure in both lateral sides of the ribbon.
It is also possible to use a molten film forming method, in which a molten web material is cast onto a quench roll. In U.S. Pat. No. 4,310,295 (corresponding to JP-B 62-38133 and JP-B 63-57222) discloses a device for uniform web pinning in which a vacuum shoe housing is divided by a baffle into a primary area and a secondary area. Vacuum forces acting on the secondary zone do not cause air turbulence in the primary zone, so that molten web edges can be prevented from wobbling.
In the method for preventing wobble by providing the pressure reduction area, however, more decompression is required in order to increase the traveling speed of the support. In that case, high vacuum force is generated to produce more air turbulence in both lateral sides, which causes nonuniformity in edge portions of the ribbon. Moreover, in the method to provide side decompression areas, the pressure in the side decompression area is fluctuated when the ribbon is vibrated. Thus, the plainness of the film is decreased to damage the appearance of the products, and to have a serious influence on the subsequent processes such as film deformation process and film conveyance process.
In view of the foregoing, an object of the present invention is to provide a method for forming a film with good plainness by decreasing air turbulence and reducing vibration of the ribbon. And another object of the present invention is to provide various kinds of devices, such as a polarizing plate protection film and a liquid display crystal device, by use of a film which is produced by the method of the present invention.
To achieve the above objects, the film formation method to form a film by casting a ribbon on a support from a flow cast die while pulling the ribbon toward the support by providing a decompression area, satisfies the following formulae:
0 less than (PCxe2x88x92PL)xc3x97100/|PC| less than 15;
0 less than (PCxe2x88x92PR)xc3x97100/|PC| less than 15;
|PLxe2x88x92PR|xc3x97100/|0.5(PL+PR)| less than 10;
wherein PC is a degree of decompression in a middle portion of the decompression area, PL a degree of decompression in said left portion of the decompression area, and PR a degree of decompression in said right portion of the decompression area.
In the preferable embodiment, a decompression device for providing the decompression area is comprised of an outer seal extended in a direction parallel to a lateral direction of the ribbon, an inner seal provided in parallel to the outer seal with a certain interval, a pair of lateral end seals attached to both lateral end portion of the inner seal, inner side seals provided between the lateral end seals, a seal plate fixed on upper ends of the lateral end seals and the inner side seals, a pair of openings formed through the inner seal between the lateral side seal and the inner seal. The degrees of decompression PC, PL, PR can be changed by varying the size of the openings. The degree of decompression in the middle area is xe2x88x921471 to xe2x88x920.98 Pa.
The lateral end seal has a thickness of 3-10 mm, the method satisfying the following formulae:
L less than (t/2)xe2x88x920.5 mm;
|LLxe2x88x92LR| less than 1.0 mm;
xe2x80x830.01 less than (S1/S2) less than 0.1;
wherein L is a length from an end of a nozzle of the flow cast die to a center line of the lateral end seal with respect to a direction perpendicular to a casting direction of the ribbon, LL is a length from the center line to a left edge of the nozzle, and LR is a length from the center line to the right edge of the nozzle, S1 is an area of a first gap below the nozzle that is surrounded by the ribbon, the support, the lateral end seal, and a perpendicular line to the support from the front end of the lateral end seal, and S2 is an area of the second gap that is surrounded by the support, the perpendicular line, the bottom edge of the lateral end seal, a line from the bottom end of the inner seal to the bottom end of the outer seal, and a line extended from the outer edge of the outer seal to the support.
A clearance C1 between the support and the lower end of the lateral end seal is more than 0.1 mm and less than 1.2 mm, and an error of the clearance C1 is within xc2x11.0 mm. A clearance C2 between the nozzle of the die and the front end of the lateral end seal with respect to a casting direction of the ribbon is equal to or less than 0.5 mm.
The ribbon is made from a solution including equal to or more than 10 wt. % cellulose triacetate, the solution being solved in a solvent including 20-90 wt. % methyl acetate, 5-60 wt. % ketones, and 5-30 wt. % alcohols. The solution includes at least one kind of plasticizer of 0.1-20 wt. % to cellulose triacetate, at least one kind of ultraviolet ray absorbent of 0.001-5 wt. % to cellulose triacetate, at least one kind of finely divided powder of 0.001-5 wt. % to cellulose triacetate, at least one kind of release agent of 0.002-2 wt. % to cellulose triacetate, and at least one kind of fluorosurfactant of 0.001-2 wt. % to cellulose triacetate.
Two or more kinds of solutions including cellulose triacetate may be applied on the support at the same time. The film obtained by the above method is suitable for a polarizing plate protection film, which is used for a liquid crystal display device.